This invention relates to a method for structuring the surface of a press plate or of an endless strip, whereby a surface structure is produced by applying a mask for partial passivation by means of a device, and the subsequent chemical surface treatment, as well as a device for the application of the method.
In the prior art, press plates and endless strips are manufactured by giving a pretreated sheet a structured surface by means of a screen printing process or a photosensitive layer and subsequent etching, for example. The screen printing is applied to the large-format sheet and then the sheets are subjected to a surface etching. All the areas that form the raised structured surface are thereby covered by a mask, so that the surface is etched only in the areas that can be directly attacked by the etching fluid. The etched out areas then form the profile valleys of the desired structure. After etching, the surface is cleaned and in particular the mask is removed so that the surface can be subjected to a further coating process, for example a hard chromium plating, by additional operations.
Alternatively, a photo process can be used in which first a photosensitive layer is applied. This layer must then be illuminated, using the mask provided, for the production of the structured surface. Then a development of the photosensitive layer is necessary, and in the meantime extensive rinsing operations are necessary so that the surface can be prepared and cleaned for the subsequent operations. After the development of the photosensitive layer, the result is a mask which can be used as a template for the etching process. In this process, therefore, first a lacquer that covers the entire surface is applied, this surface is then illuminated, the unhardened components are removed and only then can the developed mask be used for the etching of a structure. The reproducibility of the masks produced in this manner is very difficult and problematic, because the negative or positive with respect to the illumination of the light-sensitive layer must always be positioned exactly in the same position relative to the photo-sensitive layer, if more than one illumination and etching process are to be carried out in sequence, to create complex three-dimensional structures on the surface of a press plate. This is no longer possible, for example, when the negative or positive for the illumination of the light-sensitive layer is applied directly to the light-sensitive layer and the negative or positive is not at exactly the same distance from it at each point of the photo-sensitive layer. The reproducibility of the application of the mask is therefore of particular importance, especially when the photo process is used, to achieve a high image accuracy. The problems can thereby be increased if a three-dimensional structure is to be achieved by a plurality of illumination and etching processes that are carried out one after another, and for that purpose a plurality of masks have to be applied one after another, whereby an etching process is carried out between the applications of each two successive masks. As a result of the need for accurate positioning and the required number of corresponding masks, the production of the press plates or endless strips is very complex and expensive. The resolution of the masks that are applied is thereby dependent to a very great extent on the method used, in addition to which a significant number of process steps are necessary, which entails complex handling, in particular on account of the size of the press plates or endless strips.
It is also known from the prior art that, instead of a screen printing process, a mask can be created by the application of a wax which is chemically resistant to the etching agent used and thus allows an etching in the areas in which the surface is not covered by the wax. In this method, a printing head which sprays the wax on the surface is moved along an X axis and a Y axis to apply the required structure by a series of individual operating procedures. In this method, the mask is therefore applied directly to the press plate and without any intervening processes via the illumination, development and removal of the undeveloped parts of the lacquer. Theoretically, the print heads used can be constructed in a manner similar to those of an ink jet printer, except that instead of an ink a wax is sprayed which hardens on the surface of the press plate or endless strips, after which the necessary etching can be performed. Although enormous progress can be achieved with this method described in the prior art, it has been found in practice that the removal of the wax lacquer is very problematic because the wax cannot be removed with a chemical treatment and thus the cleaning of the sheets after the etching process is an extremely complex process and can be performed, for example, only with a high pressure cleaner. This process also takes a great deal of time, and if a plurality of wax layers must be applied one after another and an etching process is carried out between the application of each two successive masks to achieve a three-dimensional structure, for example, it has no significant advantages. On account of the characteristics of the wax, there are limits to the accuracy and structural detail of the mask that can be achieved, so that in particular the resolution required for precisely detailed structures cannot be achieved. The limits are set by the purely physical adherence of the wax by wetting, because when hot liquid wax is sprayed onto the cold press plate, the wax hardens too quickly and forms incompletely wetted structures similar to a “dry joint”. Nor is heating the pressed plate successful, because in that case the liquid wax runs and reduces the resolution that can be achieved.